A network includes a Mobile Station (MS) for sending a Service Flow (SF) modification request message to a Base Station (BS) to modify an SF. The BS sends a path modification request message to a serving SF Authorization (SFA) when there is the information on the SF. The BS also sends an SF modification response message comprising a Confirmation Code ‘Reject SF not found’ to the MS when there is no information on the SF. The serving SFA sends a resource reservation request message to an anchor SFA when there is the information on the SF, and sends a path modification response message comprising a Failure Indication (FI) ‘Requested Context Unavailable’ to the BS when there is no information on the SF. The anchor SFA sends a resource reservation response message comprising the FI ‘Requested Context Unavailable’ to the serving SFA when there is no information on the SF.